Device for the optical transmission of signals

ABSTRACT

Devices for providing optical signal transmissions between a transmitter unit and a receiving unit which is mobile relative to the transmitter unit. The units are optically coupled to each other via an optical transfer medium, and thus improve transmission quality by eliminating, or greatly reducing interferences that affect the transmission quality.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to devices for optical signal transmissionsbetween a transmitter unit and a mobile receiving unit, which areoptically coupled to each other via an optical transfer medium.

Optical systems are frequently employed to transmit data and signals.Such systems are fundamentally composed of a transmitter unit and areceiver unit which are interconnected via an optical transfer medium.When the optical transfer medium is free space or air, an arrangementsimilar to a light barrier is achieved.

However, optical fibers, such as glass or synthetic fibers, are morefrequently used to guide the light. In both cases, the length of theoptical path between the transmitter unit and the receiving unit is, asa rule, constant. As a result, the amplitude of the signal received inthe receiver unit is not subject to significant variations with respectto time. This furnishes a uniform transmission quality.

In the case of transmission paths which have a variable optical pathlength between the transmitter unit and the receiving unit, the signallevel at the receiver may vary accordingly. This is, inter alia, aconsequence of the attenuation along the optical path, which may giverise to a varying transmission quality. In advanced digital transmissionsystems, in particular, this may result in an undesirable increase ofthe bit error rate.

Another disadvantage of prior art optical transmission systems is due tothe finite transit time of the light through the optical transfermedium. This transit time depends on the distance between thetransmitter and the receiver, and varies within a range of almost zerowhen the transmitter is located in the immediate proximity of thereceiver, up to a maximum value which occurs when the transmitter unitis located at that end of the optical medium which is remote from thereceiver.

When the transmitter moves along the length of the medium, starting fromthe receiver up to the end of the optical medium, the transit timeincreases. In the case of a transition of the transmitter from that endof the optical medium which is remote from the receiver, to the mediumclose to the receiver, initially the light passes through the entirelength of the optical medium, resulting in a long transit time prior toarrival at the receiver. When the path length is short, on the one hand,the light arrives at the receiver almost without any transit time. Thisabrupt difference in transit time, which may occur during thetransition, may give rise to a discontinuity in phase, restricting thebandwidth which can be transmitted, and possibly resulting intransmission errors.

Particularly, when optical signals are transmitted via an opticaltransfer medium shaped in the form of a closed curve, an overlapping atthe beginning and the end of the optical medium is unavoidable unless afailure in transmission can be accepted in this position. That is, twosignals are superimposed in the receiver, at the beginning andsimultaneously at the end of the medium. The first signal reaches thereceiver after a short path, and thus also after a short time. Thesecond signal passes over a longer distance, and thus arrives at thereceiver with a substantial delay. Both signals are now superimposed andproduce an incorrect cumulative signal. As a result, the transmission isadversely affected. Specifically, with high frequencies where the signaltransit time corresponds to one half of the period, the signal isextinguished. In such a case a sensible data transmission is no longerpossible.

It is therefore an object of the present invention to improve a devicefor optical signal transmission between a transmitter unit and a mobilereceiving unit (that is, moveable relative to the transmitter unit),which are coupled to each other via an optical transfer medium, suchthat interferences upon the transmission quality may be largelyeliminated.

Another object of the invention is to provide an arrangement, in whichthe transmission quality is independent of relative movements betweenthe transmitter unit and the receiving unit.

Still another object is to provide such an arrangement in which nosignal overlapping occurs at the location of the receiving unit whichcould interfere with the data transmission. Finally, another object ofthe invention is to provide a device which requires a small space,involves moderate costs, and is specifically appropriate for wide-bandsignal transmission.

These and other objects and advantages are achieved by the invention,which is based on the proposition that, a desired independence of thebandwidth of the signal transfer times can be achieved only when thesignals are prevented from arriving at the receiver along several pathswith different transit times. This means that the independence of thebandwidth of the signal transit times is ensured if only a single signalreaches the receiver. This may be the case, for instance, on a linearpath. Independence may equally be achieved when several signals arriveat the receiver, but all signals have the same transit times to thereceiver. In the current object of the invention, both features arecombined.

In the apparatus according to the invention the light beam propagateswithin the optical transfer medium such that either optical signalsprogress along different paths within the transfer medium, so that theyarrive concurrently at the location of the receiving unit and can thusbe combined to form a single signal, or the transfer medium is designedsuch that a separate three dimensional signal transmission of theindividual light signals is ensured in order to avoid signaloverlapping.

In accordance with the objects of the invention, the curve of theoptical medium or the transfer medium, respectively, is severed at onelocation and closed as free of reflections as possible. This separatingpoint is located at that site of the curve from where the signal transittimes in all directions of propagation to the receiver are equal. Hence,the light arrives at the receiver along both paths when the transmitteris positioned above the separating point. Here, the signal transit timesare precisely equal and signal distortion does not occur. At all othertransmitter positions the light progresses along one path to thereceiver and along the other path to the separating point where it isabsorbed. Hence there is only one light path from the transmitter to thereceiver. With such a provision signal transmission over a substantiallylarger bandwidth is possible.

The inventive relates equally to an optical signal transmission betweenmoving parts. The movement may here be circular, linear or along anyother optional curve on the condition that sufficient signal couplingfrom the transmitter unit to the optical transfer medium is ensured. Inthe event of linear travel of parts, the term “path length of travel”denotes that length of the path along which the transmitter unit and thereceiving unit may be moved relative to each other. In the event ofcircular movements, it denotes the corresponding part along theperiphery of the circle. At maximum, however, it denotes the completecircumference of the circle. The same applies also to any other curvealong which a movement may be carried out.

Pursuant to the object of the invention, in order to allow for a simplelow-cost implementation of the amplifiers in the receiving unit, theoptical path length must be as short as possible. Moreover, opticalsignals with different transit times must be definitely prevented fromarriving at the receiving unit in order to achieve a high bandwidth intransmission.

Of note, due to the shortness of the optical medium, the transmissionbandwidth is also substantially wider in the case of reception ofseveral signals with different transit times. This is because thetransmission bandwidth is inversely proportional to the length of theoptical medium.

Another important aspect of the present invention is the fact thatoptical transmitters can be produced at low costs while opticalreceivers are very complex and expensive due to the wide-band amplifiersthat are used.

In one embodiment of the present invention, a device for optical signaltransmission between a transmitter and a mobile receiving unit which arecoupled to each other via an optical transfer medium, is configured suchthat the transmitter unit includes at least one optical transmitter,which transmits light signals via at least one transfer medium along atleast two paths to the receiving unit. The paths extend such that thetotal path length between the transmitter and receiving units isapproximately constant. Furthermore, the receiving unit is designed sothat due to summation of the light signals from the different paths, itwill receive a cumulative signal. The cumulative signal is approximatelyindependent of the travel time/path between the transmitter andreceiving units.

The optical signals are transmitted from the transmitter unit to thereceiving unit along at least two paths. Both signal paths are designedso that the cumulative optical path length remains approximatelyconstant and hence independent of the travel. This may be achieved in asimple manner, e.g., when an optical transfer medium such as a glassfibre of constant length is used which has ends leading to the receivingunit and which permits the coupling-in of light from the transmitterunit at any location desired. The receiving unit is designed so that itreceives the signals along the optical paths and generates a cumulativesignal by summation which is largely independent of the travel pathbetween the transmitter unit and the receiving unit.

In another embodiment of the invention, the receiving unit includesseveral optical receivers which convert the optical signals intoelectrical signals. At least one optical receiver is associated witheach optical path. The electrical signals of the receivers are thentotaled in an adjoining adder.

In another embodiment of the invention, the receiving unit is providedwith an optical adder which adds up the optical signals of the paths.After such a summation the cumulative optical signal may be convertedinto an electrical signal whenever this will be necessary.

In still another embodiment of the invention, the transmission qualitycan be even further enhanced by means of a selector preceding the adder.When more than two optical paths are available, a logic selectorfunction determines the sub-set of the best signals from these paths.The signal amplitudes, the signal-to-noise ratio, the distortions orother telecommunication signal parameters can optionally be applied asselection criterion. The logic selector function controls the selectorin a way that these signals will only be supplied to the adder forsummation.

In still another embodiment a light-conducting fibre is used as thetransfer medium. Similarly to the prior art, this fibre may be designedas glass fibre, synthetic fibre or a fibre of any other light-conductingmaterial. Moreover, the transfer medium may be a molded light-conductingbody. A light-conducting liquid is equally suitable for application asthe transfer medium.

According to another feature of the present invention, a linear designof the transfer medium is included. This is to safeguard against linearmovement between the transmitter unit and the receiving unit. Further,the linear design is preferably arranged in parallel with the directionof movement.

In a further embodiment of the present invention, in the event of acircular movement between the transmitter unit and the receiving unit,the transfer medium additionally consists of a circular configurationand is preferably disposed in parallel with the direction of movement.Moreover, the transfer medium may consist of a light-conducting fibre ina circular configuration which is doped with a fluorescent dye, so thatlight may be coupled into the fibre at any desired location.

In still another embodiment of the present invention, the transfermedium is discontinuous at least at one location, from which the transittimes of the optical signals in both directions of the transfer mediumto the receiving unit are equal.

In a particularly advantageous embodiment of the invention, the opticalsignals are converted into electrical signals via two optical convertersat the receiver site. To this end, the optical medium is interrupted atthe receiver site and an optical converter is inserted in each of thebranches. Interruption in this sense does not necessarily mean amechanical severing of the medium. Rather, it must be ensured that themedium is optically separated so that a transition of light from onebranch into the other branch will be subjected to a strong attenuation.

The two signals of the optical converters are superimposed via anoperational circuit, which may consist of an analog adder or also adigital operational circuit. When the links between the opticalconverters and the operational circuit equally present inherent transittimes or when these optical converters present different transit times,this fact must also be considered in the positioning of the separatingpoint in the transfer medium. This is in order to achieve equalcumulative signal transit times along both paths from the separatingpoint to the operational circuit.

In still another embodiment of the present invention, the optical mediumis designed such that a slight overlapping of the two branches of theoptical medium occurs at the separating point, or at both separatingpoints if two of them are provided. This provision ensures thetransmission of light from each transmitter unit to the receiver unitfrom any point on the curve. In any case, the overlapping site must bedesigned so that a perfect separation of the curve branches will beensured, and light cannot transit from one curve branch to the otherbranch. The transfer medium is preferably a light-conducting fiber,which may be designed as glass fiber, or even synthetic fiber.

Another embodiment is achieved by doping the fibre with a fluorescentdye so that it is particularly simple to couple light into the fibre atany position of the transmitter unit along the curve.

In an even further embodiment of the present invention, the receivingunit comprises at least one optical receiver which is associated with anoptical transfer medium having a length shorter than the path coveredfrom the optical transmitter relative to the transfer medium.Additionally, the transmitter unit includes at least two opticaltransmitters which are spaced from each other, along the direction oflongitudinal travel, such that the light of at least one opticaltransmitter will be coupled into the transfer medium.

In accordance with the objects of the invention, a short optical mediumis employed which covers only one part of the path length, rather thanan optical medium into which a transmitter may couple light along theentire traveling path. In the transmitter unit, several opticaltransmitters are provided so as to allow for an optical transmissionover the entire wavelength. These transmitters are arranged such thatthe optical medium will be continuously illuminated by at least oneoptical transmitter. In this way, a continuous signal transmission alongthe entire path length becomes possible.

In an embodiment of the present invention, the receivers of thereceiving unit are arranged approximately in the center of the segmentsof the optical media, rather than at the end of the segments of theoptical media (which has so far been common). With this provision thetransit times of the optical signals from both ends of the opticalmedium are equal. As a result, there is no superimposition of opticalsignals with different transit times which may result in signaldistortion and a restriction of the bandwidth.

The optical transmitters of the transmitter unit are arranged such thatthe distances between them are just wide enough so that as soon as atransmitter leaves an optical medium, a second transmitter preciselyapproaches this optical medium on the other side. This provision allowsfor continuous signal transmission. Hence, it is precisely at this pointthat two optical transmitters couple light into the optical medium.Because the two paths from the optical transmitters to the opticalreceiver of the receiving unit are equal in length, there are nodistortions whatsoever caused by differences in the signal transittimes.

In another embodiment of the invention, the receiving unit comprisesseveral optical receivers which are each connected to an optical medium.The receiving unit is designed such that the signals of the opticalreceivers are combined with each other and a higher signal level or ahigher reliability may be achieved by redundancy. It is equallyconceivable that the signals from several optical receivers are addedfor achieving a higher cumulative signal level and reduced noise. It isalso possible to combine several signals in order to permit a redundanttransmission so that when one transmitter, an optical medium or even areceiver fails, it is still possible to transmit the signals via anotherpath.

In a further embodiment of the present invention, the transmitter unitcomprises a position sensor which determines the particular opticaltransmitter which is instantaneously located above an optical medium.This information is then signaled to the corresponding opticaltransmitter. With this provision, the optical transmitter can activatethe full transmitting power and transmit optical signals. When it leavesthe range of the optical medium, this exit is signaled to it and it canreduce or even completely deactivate its transmitting power. Such anarrangement serves to reduce the total power consumption of thetransmission system. Due to deactivation of the transmitters theirservice life is extended, while the creation of electromagnetic noise inthe high-power transmitter drivers is reduced.

In even another embodiment of the invention, the receiving unitcomprises several independent optical receivers with a separate opticalmedium. The transmitter unit includes at least as many opticaltransmitters as there are signal channels. The transmitter unit and/orthe receiving unit is thus designed such that it includes an additionalselector which is controlled via a position sensor. The position sensorcommunicates to the selector switch which among the optical transmittersare instantaneously able to transmit signals via the optical medium andthe associated receiver to a specified logic signal channel. What isimportant here is the fact that each signal channel is transmitted via adefined path. Depending on the position of the transmitter and receivingunits the transmission path may vary. It must only be ensured that thesignals on a particular channel are actually transmitted on thetransmitter side to the same channel on the side of the receiving unit.

The mode of operation should be represented here again with reference toa simple example where one selector switch is provided on the side ofthe transmitter unit. If, for instance, a first transmitter 1 is locatedabove a first receiver 1 the first logic signal channel is actuallygated through from the selector switch to the first transmitter. Whenthe device now moves further by a certain distance a second transmitter2 will be above the first receiver , at “t” a later point of time. Now,the selector switch switches the signals of the first signal channel tothe second transmitter so that the latter can transmit its signals tothe first receiver again. Upon a further movement of the entirearrangement through a defined distance, a third transmitter is locatedabove the first receiver at a later point of time. Then the selectorswitch will switch the first signal channel to the third transmitter sothat the latter can transmit again its signals to the first receiver,The corresponding schematic applies to all other transmitters, receiversand signal channels equally.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in the following by exemplaryembodiments with reference to the drawings wherein:

FIG. 1 is a schematic view of an embodiment, according to the invention;

FIG. 2 shows another schematic view of an embodiment, including twotransmitter and receiving units, according to the invention;

FIG. 3 is an amplitude/locus diagram;

FIG. 4 represents a transfer medium with a separation point, accordingto the invention;

FIG. 5 illustrates another embodiment according to the invention,including two separating points and two optical converters;

FIG. 6 is a signal/transit time diagram;

FIG. 7 is a schematic view of another embodiment according to theinvention; and

FIG. 8 shows an embodiment, according to the invention, includingseveral receiving units.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a view of an embodiment of the invention, which consists of atransmitter unit 1 and a receiving unit 2. The transmitter unit includesat least one transmitter 3 which relays optical information via theoptical medium 4 to the receiving unit 2. The optical medium 4 presentsa constant length independent of the position of the transmitter unit 1relative to the receiving unit 2.

FIG. 2 is an exemplary illustration of another embodiment of theinvention. Here, as previously, the optical medium is arranged such thatthe entire length of the optical path is constant. As a result of thesubdivision of the optical medium into two parts 4A and 4B, at least twotransmitters 3A and 3B are correspondingly required. Both transmitterstransmit the same information at the same time. Moreover, shown are twooptical receivers 5A and 5B which receive the optical signals of theoptical medium, for example.

FIG. 3 illustrates, for example, the effects of the addition of twosignals on the amplitude of the cumulative signal. The position of thetransmitter relative to the receiver is plotted horizontally in thediagram. When the transmitter unit is in the left position, for example,the signal level 10 in the first receiver 5A is lower than the signallevel 11 in the second receiver 5B, due to the long optical paths. Whenthe transmitter unit is now moved to the right, the signal level in thefirst receiver 5A rises, while the signal level in the second receiver5B falls. In sum, the graph of the cumulative signal 12 is anapproximation. This graph is approximately independent of the position.

FIG. 4 shows an embodiment of the invention consisting of a receivingunit 2 and a transmitter unit 1. The units are interconnected by anoptical medium 4 of any kind desired which is shaped to constitute aclosed curve. The transmitter unit is adapted to be moved relative tothe receiving unit along this curve. What is important here is themutually relative movement.

Likewise, the receiving unit 2 may move together with the optical medium4 relative to the transmitter unit 1. The optical medium 4 isdiscontinuous or interrupted at point ST1 from where the transit timesof the signals in both branches of the curve are of equal lengths.

FIG. 5 is an exemplary view of another embodiment of the invention.Here, two optical converters 6 and 7 are provided in the receiving unit2 which are each associated with one branch of the curve. The opticalmedium 4 is interrupted at point ST2 between the two optical convertersso that light cannot be transmitted from one branch of the curve intothe other one.

FIG. 6 clearly shows the effects of the addition of signals withdifferent transit times. Curve (a) corresponds to the original signal.The signal in curve (b) presents only slight delay relative to signal(a). The addition or superimposition of the two curves results in asignal corresponding to curve (c). This signal presents only slightdistortions and is easy to evaluate in the receiving unit. In the eventof a fairly strong delay of the second signal, e.g., the one representedby curve (d), an entirely different situation occurs. The result isplotted in curve (e). The development of the curve can no longer beunambiguously interpreted. Evaluation becomes particularly complicatedin an arrangement that corresponds to the prior art as the signal shapemay vary over wide ranges (i.e., as a function of the position of thetransmitter relative to the receiver). For instance, the signal shapemay have any shape between curves (c) and (d) as a function of theposition.

FIG. 7 is a view of still another embodiment of the invention,consisting of a transmitter unit 1 and a receiver unit 2. The units areconnected to an optical medium 4. The transmitter unit includes several,but at least two, optical transmitters. Some of the optical transmittersare illustrated, for example, as 3A, 3B, 3C, 3D, and are designed sothat they are suitable for coupling optical information into the opticalmedium. These transmitters are arranged such that at least onerespective transmitter will couple the signals into the optical medium.The position sensor P determines the position of the opticaltransmitters and signals the position above an optical medium to thetransmitters in such a way that the transmitters may then activate theirtransmission power.

FIG. 8 illustrates an exemplary embodiment. Here, the transmitter unitincludes a selector switch A which, based on the information from theposition sensor P, establishes the logic relationship between the logicsignal channels, transmitters and receivers. The receiving unit 2comprises several optical receivers (5A, 5B, 5C) with the associatedoptical transfer media (4A, 4B, 4C). Here, some are shown for exemplarypurposes. However, at least one channel is provided for each logicsignal channel.

What is claimed is:
 1. A device for transmitting optical signals betweenunits movable relative to each other comprising: an optical signaltransmitting unit comprising an optical transmitter for transmittingoptical signals; an optical signal receiving unit comprising at leastone optical receiver for receiving said optical signals; and an opticaltransfer medium for coupling the transmitting unit and the receivingunit; said optical signal transmitting unit being movable along saidoptical transfer medium relative to said signal receiving unit; whereinsaid optical transfer medium is shaped as a closed curve and has a firstdiscontinuity at a position from which two optical signal paths to saidoptical signal receiving unit are formed and from which the transittimes of optical signals along each of said two optical signal paths tosaid optical signal receiving unit are equal.
 2. The device according toclaim 1, wherein said optical signal receiving unit further comprisestwo optical converters for converting the optical signals from eachrespective optical path to electrical signals, and an operationalcircuit for electrically combining said electrical signals.
 3. Thedevice according to claim 2, wherein said transfer medium has a seconddiscontinuity between said two optical converters such that each opticalconverter receives light only from an associated optical path.
 4. Thedevice according to claim 1, wherein said transfer medium is formed sothat said optical paths slightly overlap at said first discontinuity toensure as optical transmission from any point along said transmissionmedium.
 5. The device according to claim 1, wherein said transfer mediumis closed-off at said first discontinuity without an reflection of lightoccurring.
 6. The device according to claim 1, wherein said transfermedium is a light conducting optical fiber.
 7. The device according toclaim 6, wherein said light-conducting optical fiber is doped with afluorescent dye.